Refrigeration systems commonly employ a compressor throttling valve set to a fixed pressure setting to limit the load on the compressor prime mover. The throttle valve is set to limit the pressure and the load on the prime mover for the worst case condition, which is during a hot gas defrost mode. The defrost setting penalizes the cooling capacity of the refrigeration system, as the restriction in the suction line presented by the throttle valve is present at all times.
When the compressor will be driven by a selected one of two prime movers, such as in a transport refrigeration system which may be driven by an electric motor when an associated truck, trailer, or container is stationary and near a source of electric potential, and otherwise by a Diesel engine, the worst case condition takes into account the smaller of the two power ratings. Thus, the pressure setting of the throttling valve is set for the horsepower of the electric motor and the normally greater power available from the Diesel engine is not usable.
Co-pending application Ser. No. 304,686, filed Feb. 1, 1989, entitled "Transport Refrigeration System With Improved Temperature And Humidity Control", now U.S. Pat No. 4,899,549 which is assigned to the same assignee as the present application, discloses a suction line modulation valve and associated modulation control. The modulation control controls the modulation valve to restrict the suction line during heating and cooling modes near the set point temperature, according to a predetermined control algorithm, with the valve otherwise being open. The normal compressor throttling valve is eliminated, with a prime mover overload condition causing the modulation control to control the modulation valve to restrict the suction line and reduce the pressure, thus reducing the load on the prime mover.